Generally, hydrotreating reactions involve the application of hydrogen to a substrate, usually under elevated temperature and pressure, in the presence of a catalyst with the target of causing a physical or chemical change in the substrate. Most such hydrotreating reactions occur in refinery operations where the substrate is a hydrocarbon feedstock.
Conventional hydrotreating catalysts are generally in the form of a carrier of a refractory oxide material on which metal compounds providing the hydrogenation components have been deposited, the choice and amount of each component being determined by the intended use. Refractory oxide materials common in the art are amorphous or crystalline forms of alumina, silica and combinations thereof. These oxide materials can have some intrinsic catalytic activity but often only provide the support on which active metals compounds are present.
The metals are generally base or precious metals from Group VIII and Group VIB of the Periodic Table which are deposited in oxidic form during manufacture; in the case of base metals, the oxides are then sulphided prior to use to enhance their activity.
According to the present state of the art, catalysts based on molybdenum or tungsten together with either cobalt or nickel as promotor are employed for the removal of sulfur and nitrogen compounds and of metals out of crude oil fractions. Such catalysts are known as hydrotreating catalysts. Almost universally alumina is employed as a support for such hydrotreating catalysts. Alumina-supported catalysts can be easily shaped by extrusion and provide after calcination mechanically strong bodies.
Hydrotreating of crude oil fraction is performed in two types of reactors. The first reactor type is known as ebullating bed reactors. The catalyst bodies are kept floating in a stream of liquid and gas. The flow is controlled thus that the catalyst bodies remain floating and are neither entrained with the upwards flow of the liquid nor settle to the bottom of the reactor.
The other type of reactor is employing a trickle flow process. A gas flow and a liquid flow are passed downwards through the reactor. The reactor is mainly filled by the gas phase, while the liquid crude oil fraction to be treated flows as a thin layer over the catalyst. Both types of reactor are calling for mechanical strong catalyst bodies. With the ebullating bed, the catalyst bodies are moving in the liquid flow, which readily leads to attrition with catalyst bodies of an insufficient mechanical strength. With the trickle flow process the catalysts are used in very large fixed-bed reactors.
Since the weight of the catalyst bed is considerable, the catalyst bodies have to be very strong to avoid fracturing of the catalyst bodies within the lower sections of the catalyst bed. Fracturing of the catalyst bodies leads to a rise in pressure drop of the gas flow passed through the fixed bed, which is technically not acceptable.
Alumina can provide a high surface area and a high bulk density, which is important for the amount of catalyst that can be loaded in a reactor of a given size, and can be readily extruded to bodies that get a high mechanical strength after calcination. Usually alumina produced from boehmite (AlOOH) or pseudo-boehmite is required to provide optimum results. Pseudo-boehmite exhibits needle-shape crystallites as boehmite, but the crystallites are much smaller. The shape of the alumina crystallites is maintained during calcination, but the alumina reacts to small crystallites of cubic γ-alumina, or depending on the calcination procedure to other transition aluminas.
In spite of the favorable properties of γ-alumina as a support for hydrotreating catalysts, the support exhibits some significant drawbacks. First of all cobalt and nickel are liable to react with the transition alumina to a spinel, cobalt aluminate or nickel aluminate, thus leading to a loss in any significant activity in the previously described hydrotreating reactions. Treatment with hydrogen or hydrogen and hydrogen sulfide does not release the majority of the cobalt and nickel from the aluminum spinel. Presumably molybdenum and tungsten react also with the alumina surface. The interaction leads to the supported molybdenum oxide and tungsten oxide more difficult to reduce and to react to Mo(IV)S2 and W(IV)S2.
Another drawback of alumina-supported hydrotreating catalysts involves the processing of spent catalysts. Though molybdenum and cobalt are expensive, it is difficult to reclaim the metals from the alumina support. Dissolution of the alumina support in alkali calls for much alkali and leads to a foaming solution. The molybdenum dissolves together with the aluminum in contrast to the cobalt, which is deposited as cobalt hydroxide. Filtration is difficult as well as the separation of the resulting molybdate from the aluminate solution. Generally spent hydrotreating catalysts are therefore disposed.
A final problem with alumina-supported hydrotreating catalysts is that the pore-size distribution of the catalyst cannot be adequately controlled. With catalytic reactions involving a solid catalyst and a liquid phase, the transport through the pores of the catalyst proceeds slowly. Catalyst bodies within a fixed-bed reactor cannot be made smaller than about 1 mm, since a smaller size leads to an unacceptably high pressure drop. With the necessarily large catalyst bodies, the pore length is thus that only the external edge of the catalyst bodies contributes significantly to the catalytic reaction. It is therefore essential to employ catalyst bodies with wide pores. However, a high pore volume, which is characteristic for wide pores, leads to a low mechanical strength. With the compromises that has to be settled between mechanical strength and width of pores, the mechanical strength is most important. The usual alumina-supported hydrotreating catalysts therefore exhibit a high mechanical strength and relatively narrow pores.
Particularly with the removal of metals from crude oil fractions, the relatively narrow pore mouths of usual alumina-supported hydrotreating catalysts are a drawback. Metals removed out of the crude oil fraction are deposited within the mouths of the pores, which leads to a rapid blocking of narrow pores and therefore deactivation of the catalyst.
A high pore volume is therefore attractive for hydrotreating catalysts. However, a high pore volume is difficult to combine with a high mechanical strength of the catalyst bodies. Since hydrotreating catalysts are employed in very large fixed bed reactors, the weight of the catalyst bed asks for a high mechanical strength of the catalyst bodies.
A support that can provide by extrusion catalyst bodies with an elevated pore volume of the same mechanical strength as alumina or even a higher strength without reacting with cobalt or nickel would be extremely valuable.
An alternative for alumina as a support is silica. However, silica is notoriously difficult to shape by extrusion. The dies are rapidly worn out by silica. Furthermore the bulk density of silica is significantly lower than that of alumina. Finally application of suitable molybdenum or tungsten compounds finely divided and uniformly distributed over silica surfaces is difficult. Silica-supported hydrotreating catalysts are consequently not useful.
Titania has already been proposed as a support for hydrotreating catalysts. Titania is generally produced by reaction of ilmenite, iron(II) titanate, FeTiO3, with sulfuric acid. Dilution of the titanium sulfate solution leads to hydrolysis of titanium(IV) to hydrated titanium dioxide. The very small titanium dioxide particles are highly hydrated. Removal of the water leads to severe sintering of the titanium oxide to relatively large titania particles. The specific surface area of the titania is less than 50 m2 per gram, which is considerably lower than that of the alumina employed as support for hydrotreating catalysts, viz., from about 130 to 250 m2 per gram.
An alternative production procedure of titania proceeds via titanium tetrachloride. The titanium tetrachloride is brought into a hydrogen-oxygen flame, where it reacts to titania particles of about 30 nm. The resulting (pyrogenic) titania particles are always a mixture of the Anatase and Rutile phase of titania and exhibit a surface area of about 50 m2 per gram and a pore volume of about 0.50 cm3 per gram. The Rutile phase of titania is in general considered as detrimental for any catalyst formulation involving base metals as the Rutile phase very easily incorporates those metals by formation of metal titanates, which are not catalytically active. Indeed the reaction of metal oxides like Nickel oxides and Chromium oxides leads to the formation of yellow pigments, but not to active catalysts.
The effect of titania on the catalytic activity of hydrotreating catalysts has not been established unambiguously. First of all, the surface area of a titania support that is commercially available pyrogenic titania, has a surface area of about 50 m2 per gram, which is considerably lower than the surface area of the γ-alumina, which is usually employed for the production of hydrotreating catalysts, viz., about 200-300 m2 per gram. Another difference is that nickel and cobalt are promoting the activity of alumina-supported molybdenum and tungsten catalysts and have no significant effect on the activity of titania-supported catalysts.
In 1989, McCormick et al. investigated the influence of the support on the performance of coal liquid hydrotreating catalysts [Robert L. McCormick, Julia A. King, Todd R. King and Henry W. Haynes Jr. Ind. Eng. Chem. Res. (1989), 28, 940-947]. The authors studies a range of different supports, among which alumina, titania, and titania-alumina. The alumina-supported catalyst had a surface area of 167 m2 per gram, a pore-volume as measured by mercuri porosimetry of 0.75 cm3 per gram, and a pore volume measured by the condensation of nitrogen at 77 K (small pores) of 0.43 cm3 per gram. The titania-supported cobalt-molybdenum catalyst had a surface area of 68 m2 per gram a pore volume of 0.27 cm3 per gram and a volume of small pores of 0.11 cm3 per gram. A titania-alumina support was prepared by coprecipitation of aluminum trichloride and titanium tetrachloride by ammonia. Nickel and molybdenum was applied on the titania-alumina support. Whereas the alumina-supported and the titania-alumina supported catalysts exhibited a high activity and an excellent maintenance of the activity, the titania-supported catalyst displayed a poor activity and a low stability.
Prins et al. performed a careful and extensive study on cobalt-molybdenum and nickel-molybdenum catalysts supported on alumina, titania and carbon. The titania support had a surface area of 46 m2 per gram and a pore volume of 0.5 cm3 per gram, while the alumina showed a surface area of 233 m2 per gram [S. P. A. Louwers, M. W. J. Crajé, A. M. van der Kraan, C. Geantet, and R. Prins, J. Catalysis (1993), 144, 579-596]. The activity of the alumina-supported catalyst was a factor of about 3.6 higher than that of the titania-supported catalyst.
Lecrenay et al. performed a study dealing with titania-alumina supports [E. Lecrenay, K. Sakanishi, T. Nagamatsu, I. Mochida, and T. Suzuka, Applied Catalysis B Environmental (1998) 18 325-330]. The authors studied the hydrodesulfurization of a number of model compounds, but also that of gasoils and light cycle oil. A pure alumina support was employed and two commercially produced titania-alumina supports by hydrolysis of titanium and aluminum alkoxides. The surface area of the catalysts were between 240 and 252 m2 per gram and the pore volumes between 0.54 to 0.60 cm3 per gram. Nickel and molybdenum and cobalt-molybdenum were applied on the supports. It is important that the activity for the hydrodesulfurization of gasoil dropped with the titania content and that of light cycle oil was with 8 wt. % titania about the same as that of pure alumina, while that with 25 wt. % titania was lower. It is therefore apparent that the effect of addition of titania to alumina for the hydrodesulfurization of gasoil and light cycle oil is not significant.
Concerning pure titania supports, Inoue et al. published a paper on a novel preparation procedure of titania supports [S. Inoue, H. Kudou, A. Muto, and T. Ono Fuel Division Preprints (2003) 48(1) 88-89]. The procedure employed was extensively described in U.S. Pat. No. 4,422,960 to Chiyoda Chemical Engineering and Construction Co., Ltd., Yokohama, Japan. The objective of the procedure is to produce supports of a volume of wide pores and a substantial surface area. The principle is based on controlled Ostwald ripening; small particles of the support are grown to larger particles under conditions that avoid the formation of smaller particles. Fairly uniform particles are thus obtained of a size leading to a high volume of large pores. The procedure has been applied to alumina, silica, titania, and sepiolite. The surface areas of thus produced titanias are between 133 and 175 m2 per gram and the pores have a fairly uniform size varying with the size of the elementary titania particles between 6 to 20 nm. However, the pore-size distribution must be described by a fairly complicated procedure and an improved strength of the titania bodies has not been mentioned.
WO 2004/073854 describes a catalyst composition which comprises one or more Group VIB metals, one or more Group VIII metals, and a refractory oxide material which comprises 50 wt % or more titania, on oxide basis, which is prepared by precipitation techniques, and the use thereof in the hydroprocessing of hydrocarbonaceous feedstocks.
An alternative titania support can be obtained by a procedure described in EP-A1-1748033 as well as slightly differently in US 2010/0069233. The procedure involves treatment of titania, which is either amorphous, or consists of anatase, or of rutile, at temperatures from 100 to 300° C. with an alkaline solution, preferably sodium hydroxide. The reaction products have a special crystallographic structure that is different from anatase, rutile or brookite, and consists of non-uniform particles with an elongated shape, nanowires, nanofibers, or nanotubes. The material contains hydrogen or a combination of hydrogen and sodium. The titanium oxide preparation leads to relatively large surface areas, up to 300 m2 per gram, and pore volumes, up to 0.70 cm3 per gram.
Escobar et al. [J. Escobar, J. A. Toledo, M. A. Cortés, M. L. Mosqueira, V. Pérez, G. Ferrat, E. Lopez-Salinas, and E. Torres-Garcia Catalysis Today (2005) 106 p. 222-226] prepared titania in the form of nanotubes having a surface area higher than 300 m2 per gram by the above proprietary methodology. The activity was twofold that of alumina-supported cobalt-molybdenum. The surface area of the support thus produced is fairly elevated, viz., 343 and 335 m2 per gram. The pore volume varies more, viz., 0.70 and 0.47 cm3 per gram. It is striking that application of the active components, cobalt and molybdenum, strongly affected the surface area, which dropped to 181 and 174 m2 per gram as well as the pore volume, which was 0.29 cm3 per gram after the impregnation. Preparing shaped titania supports, being sufficiently catalytically active, on the basis of powderous or nanotubular materials being loaded with the active metal compounds failed due to insufficient mechanical or catalytical properties.
The above state of art documents indicate that much work has been performed to improve the properties of hydrodesulfurization catalysts by employing titania as a support. The titania-alumina supported catalysts have not indicated a clear improvement of the catalytic performance. The surface area of the pure titania support produced by flame hydrolysis of titanium tetrachloride is not sufficient, while the support according to U.S. Pat. No. 4,422,960 and the support described by Escobar et al. above exhibit after impregnation with the active components a lower surface area and pore volume. All catalyst described are powder materials tested in micro testing units.
The use of titania, or titanium dioxide, as a catalyst support for a conventional hydroprocessing catalyst is limited by the lack of a useful pore structure. Therefore the few titania-supported commercial hydroprocessing catalysts as detailed above that exist in the market have a low pore volume and as a result can hold or support less hydrogenation metals than the more common alumina-supported catalysts. Generally it is acknowledged that thermal stability, low surface area and poor mechanical strength have all hindered the commercial exploitation of titania supported catalyst systems.
It is therefore an object of the present invention to provide titania-supported hydrodesulfurization catalysts of a sufficiently large surface area, a high pore volume and a high mechanical strength.